preflights in Makefiles

Makefile

.PHONY: envdir
serve: envdir
	$(eval ENVDIR := $(shell ./preflight.sh))
	$(ENVDIR) /usr/local/myservice/env python -m SimpleHTTPServer 8000

preflight.sh

#!/bin/sh

TARGET="${1:-envdir}"
PACKAGE="${PACKAGE:-${TARGET}}"
TARGET_PATH="$(command -v ${TARGET} 2>/dev/null)"

if [ -n "${TARGET_PATH}" ]
then
	echo "${TARGET_PATH}"
	exit 0
else
	echo "${TARGET} missing, attempting to install." > /dev/stderr
	case "$(uname -o)" in
	GNU/Linux)
		sudo apt-get install "${PACKAGE}" ;;
	Darwin)
		brew install "${PACKAGE}";;
	*)
		echo "$(uname -o) isn't implemented yet; PR maybe?" > /dev/stderr ;;
	esac

	# You need to exit, otherwise all of the package output will be captured
	# in stdout.
	exit 1
fi